This invention relates to method and apparatus for feeding or delivering material suspended in a liquid medium in controlled fashion to a moving endless filter such as a porous or foraminous belt or the like whereupon the liquid medium is removed or separated as a filtrate and the suspended material remains on the foraminous carrier as a cake or residue of substantially uniform thickness or depth and substantially evenly distributed across the width of the carrier. Stated otherwise, the invention relates to method and apparatus for dehydrating a liquid suspension of material to form a filter cake by draining liquid off and/or through a moving carrier to obtain a filter cake ribbon whose width is that of the carrier and having a reasonable uniformity of thickness.
Prior art efforts in this direction have concentrated on the use of physical manipulation of the material and particularly of the incipient cake as it is being formed after it is fed to the carrier as the liquid drains or is carried away, as by the physical manipulations of spreading the material transversely across the carrier and imparting uniformity of thickness with doctors, plows or the like and ultimately by dual filter belt pressing in order to achieve a filter cake or residue which conforms reasonably well with the width of the carrier and is of relatively uniform thickness.
Without eschewing the possibility of using these prior art physical manipulations to aid in achieving the desired uniformity in thickness and width of the cake, the invention disclosed and claimed herein is directed to the delivery or feed of the suspension to a belt or like foraminous carrier such that at the inception of cake formation, a more even and uniform distribution of the suspension is presented to the foraminous carrier.
In the dewatering of a suspension to form a filter cake, as for example in the treatment of sewage sludge where the water removal is effected on an endless belt filter, it is considered desirable to flocculate the material prior to its introduction onto the filter. The reason for this is to lessen premature "blinding" of the filter, which results in inefficient dewatering. That is to say, if the material migrates or settles too rapidly and/or in randomly localized fashion onto the belt or filter, premature "blinding" of the filter may result, which retards dehydration and renders it non-uniform in nature. This result is counterproductive and should be avoided to the extent possible. Thus, it is considered that the suspension of material should be of pulpy form or in the form of a flocculated suspension. The premature "blinding" effect, however, can remain a factor if the suspension is so treated as to upset or overcome the binding forces which hold the clumps or flocs of material together prior to or during distribution of the suspension onto the filter. To the degree that premature "blinding" takes place, dehydration may occur non-uniformly, more slowly than would otherwise be the case and, in general, in such fashion that the resultant filter cake is not as uniform as to thickness and moisture content as is desired or possible. It is generally considered that the suspension should be delivered to a central region onto the belt, followed by mechanical distribution of the developing filter cake uniformly as to depth and across the width of the belt during the initial dehydration of the suspension, whereafter the initially dewatered cake may be subjected to compaction and accelerated further dehydration between two filter belts traveling in unison. The method and apparatus disclosed in U.S. Pat. No. 4,142,971 (Le Fur et al.) issued Mar. 6, 1979, is exemplary of such a system.
I have found that significant improvements are possible by controlling the distribution of suspension onto the moving filter/carrier. Specifically, it is possible to load the carrier and its supporting rolls and bearings more uniformly and evenly, and favorably affect dehydration efficiency by the method and apparatus disclosed below.
Of particular importance is the method of forming a filter cake which comprises the steps of forming a flocculated suspension of particulate material in a liquid medium, controlling the velocity profiles of the flocculated suspension while discharging such suspension unidirectionally onto an endless moving filter so that the flocs or clumps of the suspended material are minimally disturbed prior to their deposition onto the moving filter and before the onset of significant separation of the liquid medium from the suspension.
An object of the invention is to form a relatively uniform ribbon discharge of suspension to the moving carrier.
Another important feature of the invention involves the method of concentrating a suspension of material which comprises the steps of feeding an even and uniform distribution of the suspension onto and across the widthwise dimension of a traveling belt or like porous carrier, draining filtrate through the carrier as it travels, and recovering concentrated material from the carrier after its moisture content has been reduced.
Another object of the invention is to form a relatively quiescent but dynamic, overflowing pool of suspension which discharges a ribbon of suspension having a substantially uniform thickness and width onto the receiving carrier.
The invention concerns apparatus for concentrating an aqueous suspension of material, which comprises the combination of endless porous belt means for separating filtrate while retaining the material, means for driving the belt means such that a flight portion thereof travels horizontally, hopper means for containing a pool of the suspension, and means for welling the suspension into the bottom of the hopper means to form a pool of suspension within the hopper means which discharges a continuous ribbon of suspension onto the porous belt means.
Another object of the invention is continuously and quiescently to well suspension upwardly to form a relatively shallow ribbon of suspension which flows gently onto a traveling porous carrier.
Another object of the invention is to form a shallow pool of suspension by welling suspension into it and to discharge a ribbon of suspension continuously at one side of the pool and onto a traveling porous carrier.
Another important feature of the invention involves the method of concentrating a suspension of material in a liquid medium which comprises the steps of forming a relatively quiescent but dynamic, overflowing pool of the suspension, discharging the suspension from the pool in the form of a ribbon having a substantially uniform thickness and width flowing unidirectionally onto a traveling, porous carrier, and recovering concentrated material of predetermined moisture content.
Another object of the invention is continuously to well suspension into a pool having a discharge side so that the suspension flows horizontally but initially omnidirectionally to maintain the pool in continuously overflowing condition to deliver a continuous and unidirectionally flowing ribbon of suspension at the discharge side of the pool onto a suitable carrier.
Another object of the invention is to form a continuous weir overflow at one side of a flooding pool of suspension by introducing the suspension generally centrally and substantially continuously and quiescently into the bottom of the pool.
Consequently, the invention involves apparatus for concentrating an aqueous suspension of material, which comprises the combination of endless porous belt means for separating filtrate while retaining the material, means for driving the belt means such that a flight portion thereof travels horizontally, hopper means for containing a pool of the suspension and including a weir transversely above the flight portion of the belt means, and means for welling the suspension into the bottom of the hopper means to form a pool of suspension within the hopper means which discharges a continuous ribbon of suspension over the weir.
A further object of the invention is to form a column of upwardly rising and uniform suspension, well it into the bottom of a pool of suspension at a sufficiently slow rate to cause the suspension being introduced to spread omnidirectionally and then channeling the suspension to flow unidirectionally from one side of the pool as a ribbon of suspension dimensioned in thickness and width for discharge onto a moving, porous carrier.
Another object of the invention is directed to apparatus for bottom flooding a pool of a suspension, such as material flocculated in a liquid medium, for discharge in the form of a ribbon-like river of such suspension onto a moving, porous carrier such as a filter belt.
A further object of the invention is to provide a method of forming filter cake and apparatus to perform the method including a moving filter belt and means for discharging a ribbon of suspended material onto the belt, which ribbon substantially covers the width of the belt with a substantially uniform depth of suspension.
In another aspect, an object of the invention is to integrate flocculation of a suspension, such as sewage sludge in an aqueous medium, while welling such stable suspension into and flooding the bottom of a pool of the suspension which overflows substantially gently onto and in substantially width-covering relation to a moving filter belt.
A further object of the invention is to control the velocity profiles of a flocculated suspension onto a moving carrier such as a filter belt, so that the flocs or clumps of the suspended material are minimally disturbed prior to their distribution onto the belt and before the onset of significant separation of the filtrate from the suspension.
The above and other objectives of this invention will become more apparent from the following preferred embodiments of my invention as described and illustrated hereinafter.